For many years, roofing systems made of metal in various sheet gauge thicknesses have been used. Metals such as carbon steel, stainless steel, copper and aluminum are the most popular types of metal roofing systems. Carbon steel metal roofing systems are commonly treated with a corrosion-resistant coating to prevent rapid oxidation of the iron. One type of corrosion-resistant coating for carbon steel is a tin metal coating. Tin coating of carbon steel is a well-known process and has been used in various industries, especially in the food industry. Tin coating of carbon steel is normally carried out by a continuous, high-speed electrolysis process. In an electrolysis process, an electrical current is used to reduce alkaline or acidic electrolytes of tin to plate the tin on the carbon steel. The thickness of the tin coating ranges between 3.8.times.10.sup.-4 to 20.7.times.10.sup.-4 mm (1.5.times.10.sup.-5 -8.15.times.10.sup.-5 in.). The equipment and materials used to electroplate carbon steel are very expensive and relatively complex to use; however, only a thin layer of tin is used so the cost of the expensive tin is maintained quite low. A less used process of coating carbon steel is by a hot dipping process. This process is normally not used because of the resulting minute areas of discontinuity in the tin coating. Consequently, the material is less satisfactory for food containers. In addition, hot dipped tin forms a thicker coating which is prone to flaking.
Tin is an important material in that it is relatively inexpensive and highly resistant to corrosion. Corrosive materials such as carbon steel can be coated with tin to produce highly corrosive-resistant and relatively inexpensive products such as tin cans and tin roofing materials. Many metallic alloys have been developed which resist corrosion, such as stainless steel. Stainless steel is an alloy of iron and chromium and sometimes includes nickel and molybdenum. The chromium within the stainless steel alloy is the primary alloy component which inhibits corrosion. The chromium forms chromium oxide and tightly bonds to the surface of the stainless steel thus preventing oxygen from penetrating into the stainless steel to form corrosive ferrous oxides. Carbon steel has little if any chromium content, thus the iron readily oxidizes with the surrounding oxygen to form ferrous oxides commonly known as corrosion.
Although stainless steel corrodes at a significantly slower rate than standard carbon steel, the stainless steel will eventually corrode and will corrode at a significantly faster rate than carbon steel coated with tin plate. Previously, the concept of coating stainless steel with a corrosive-resistant material was unheard of since stainless steel in and of itself is a corrosive-resistant material. Furthermore, attempts to coat stainless steel have proven of limited success. Specifically, coating stainless steel with tin by a hot-dip process has repeatedly been unsuccessful using conventional hot-dip processes. The tin coating repeatedly flakes off the stainless steel soon after being coated and/or during pre-forming and installation. Until now, industrial manufacturing of hot-dipped tin coated stainless steel has been unsuccessful. Presently, the only process which semi-successfully coats stainless steel with tin is the electroplating process. The electroplating of stainless steel involves the use of very expensive and relatively complex machinery. The electroplating of tin onto stainless steel results from running a stainless steel strip through a stanneous solution. An electrical current is introduced to the stanneous solution and the tin is reduced and plated onto the stainless steel strip. The thickness of the tin plate is limited to a thickness not more than 20.7.times.10.sup.-4 mm (8.15.times.10.sup.-5 in.). The limited tin coating thickness resulting from electroplating limits the uses and life of the tin plated materials. Although tin is a highly corrosion-resistant material, tin will slowly corrode in harsh environments such as salt water or acid environments. Thicker tin coatings in such environments would vastly increase the useable life of the tin coated materials.
Coating stainless steel with tin alloys by a hot-dipped process have been more successful. One of the most popular tin alloy coatings for carbon steel and stainless steel is a tin-lead alloy commonly known as terne. The composition of the terne alloy is generally about 80 weight percent lead and about 20 weight percent tin. The lead in the terne alloy readily bonds to both carbon steel and stainless steel to form a strong and durable tin alloy coating. Although terne coated sheet metals have excellent corrosive-resistant properties and have been used in a wide variety of building applications such as roofing, terne coated materials have recently raised environmental concerns due to the lead content of the terne alloy. Although the lead in the terne alloy is stabilized, there is some concern, albeit unfounded, about leaching of the lead from the terne alloy. As a result, terne coated materials have been limited from use in various applications, such as aquifer roofing systems. Terne alloys are also a softer material than tin, thus, wear faster than tin coatings and are not as strong as tin coatings. Due to the expensive nature of electroplating stainless steel materials and the limitations as to the thickness of the stainless steel materials, there has been a demand for a process for successfully hot dipping stainless steel materials with tin.